He told TMZ: "It was a loose end that always bothered me. I just have this lifelong habit of never finishing things."

He explained that his parents Martin Sheen and artist Janet Templeton attempted to convince him to complete his diploma.

"The idea my parents had was for me go to summer school," he recalled. "My response was, 'There's school during the summer? That's crazy!'

"I could think of nothing worse in my first summer of freedom after all that time. I told them, 'Let me go on some auditions and if it doesn't work out, I'll go and get my GED [a diploma equivalency test] and go to college.'

"But I got the first job I went on. It was Grizzly II: The Predator, that horrible thing."